The present invention relates to a magnetoresistive component having a repeated geometrical pattern and to a magnetoresistive transducer using said component. This transducer is more particularly intended for producing heads for reading and/or writing informations recorded in magnetic form on a random support such as a hard disk, tape, floppy disk, ticket, card, etc.
This transducer can also be used for detecting and locating weak magnetic fields such as the geomagnetic field and the leaking of weak magnetic fields associated with the presence of cracks in metallic systems.
Magnetoresistive components are generally in the form of a small magnetoresistive bar, whose ends are connected to leads for making a current flow in the bar. In the presence of an external magnetic field, the magnetization M of the resistant bar is differently oriented having the effect of modifying the resistivity of the material. A voltage variation is then observed at the bar terminals and the measurement of this voltage variation represents the value of the external magnetic field. It can also represent its direction.
Thus, by measuring the resistance of the bar, it is possible to measure a magnetic field and conversely measure an electrical resistance by measuring a magnetic field.
The most modern magnetic transducers use magnetoresistive materials deposited in thin film form on a suitable substrate. These films are etched by photolithographic processes in order to give them the shape of a bar.
The magnetic saturation field H.sub.s is defined as being the magnetic field applied to the magnetoresistive component above which the intrinsic electrical resistivity .rho. of the component virtually no longer varies. The following ratio is defined: ##EQU1## in which H represents the value of the field applied: .rho.(H=0) being the resistivity of the magnetoresistive component in zero field and .rho.(H=Hs) its resistivity when the component is in the presence of a magnetic field equal to the saturation field. Moreover, .DELTA..rho./.rho.=.DELTA.R/R, with R representing the resistance of the magnetoresistive component.
The sensitivity .alpha. of the magnetoresistive structure and therefore that of a magnetoresistive transducer are defined by the variation of the magnetoresistance .DELTA.R/R obtained by the application of a unitary magnetic field and it satisfies the relation: .alpha.=(.differential.(.DELTA.R/R)/.differential.H).sub.H&lt;Hs.
The shape of a magnetoresistive component or rather its dimensions influence the orientation of its magnetization. Thus, in the case of a magnetic film, the magnetization tends to be aligned in the plane of the film. Moreover, in the case of a long bar, the magnetization is generally oriented in accordance with said length. This effect amounts to a search for the minimum magnetic energy.
For a given magnetization direction, in the material appears a magnetic field known as the demagnetizing field H.sub.d or form anisotropy field oriented in the direction opposite to that of the magnetization. The intensity of the demagnetizing field is a function of the geometry of the material in question and the direction taken by magnetization within it.
FIG. 1 diagrammatically shows a magnetic bar having an elliptical section, in order to illustrate the components of said demagnetizing field.
The values of the demagnetizing field coefficients are designated N.sub.X, N.sub.Y and N.sub.Z respectively in the directions X, Y and Z of an orthonormal coordinate 0XYZ. When the magnetization M is parallel to the direction X, the demagnetizing field proves the relation H.sub.d =N.sub.X. M.
For a magnetoresistive bar of elliptical section and assumed to be infinitely long and aligned in the direction X, having a thickness e in the direction Z and a height h in the direction Y, the following relations are proved: N.sub.X =0, N.sub.Y =e/(h+e), N.sub.Z =h/(h+e).
In the case of a bar with a rectangular section of thickness e small compared with its height h, which is itself small compared with its length L, a first approximation consists of considering that: N.sub.X =0, N.sub.Y =e/h, N.sub.Z =1.
Thus, an external magnetic field H applied in the direction Y will be reduced within the magnetoresistive component by the demagnetizing field H.sub.d approximately equal in the latter particular case to (e/h).M, whereas said same field applied in direction X will not be reduced.
It should be noted that this approach no longer strictly applies as soon as the component length is comparable with its height or width. Thus, the demagnetizing field is liable to increase the value of the saturation field Hs and therefore reduce the sensitivity of the transducer.
Hitherto the magnetoresistive materials used for producing magnetic transducers were ferromagnetic-type monolithic materials mainly consisting of compounds based on iron and nickel (Fe.sub.19 Ni.sub.81,Fe.sub.20 Ni.sub.80) and those based on iron, nickel and cobalt (Fe.sub.15 Ni.sub.65 Co.sub.20, Fe.sub.5 Ni.sub.80 Co.sub.15, . . . ).
At present, new magnetoresistive materials are available. These are metallic multilayer magnetic structures (MMMS) constituted by a stack of magnetic layers separated by non-magnetic metallic layers having a thickness such that there is an antiferromagnetic coupling between the magnetic layers.
The investigated metallic multilayer magnetic structures are of different types. They are constituted by metals chosen from among cobalt, iron, nickel, alloys of iron and nickel, copper, chromium, silver, gold, molybdenum, ruthenium and magnesium, as described in document (1) by H. Yamamoto and T. Shinjo (IEEE Translation Journal on Magnetics in Japan, vol. 7, No. 9, September 1992, "Magnetoresistance of multilayers", pp. 674-684).
The MMMS's at present having the largest number of requisite properties (significant magnetoresistive effect, low saturation field, low coercivity, good annealing resistance) are constituted by FeNi layers separated by copper layers and as described in document (2) by S. S. P. Parkin ("Oscillations in giant magnetoresistance and antiferromagnetic coupling in [Ni.sub.81 Fe.sub.19 /Cu].sub.N N multilayers", Appl. Phys. Lett. 60, No. 4, January 1992, pp. 512-514) and document (3) by R. Nakatani et al. (IEEE Transactions on Magnetics, vol. 28, No. 5, September 1992, "Giant magnetoresistance in Ni--Fe/Cu multilayers formed by ion beam sputtering", pp. 2668-2670) or thin FeNi layers separated by silver layers, as described in document (4) by B. Rodmacq et al., (Journal of Magnetism and Magnetic materials 118, 1993, pp. L11-L16, "Magnetoresistive properties and thermal stability of Ni--Fe/Ag multilayers").
These new materials have the property of being highly magnetoresistive, i.e. having a ratio .DELTA..rho./.rho. from 10 to 20, whilst having weak magnetic saturation fields, i.e. below 40 kA/m.
In MMMS's the magnetoresistive effect corresponds to the rotation of the magnetizations of each of the magnetic layers, which are accompanied by a reduction in the electrical resistance during the application of an external magnetic field. In this case the saturation field Hs corresponds to the magnetic field which it is necessary to apply in order to orient in the same direction and sense, the magnetization of each of the different magnetic layers.
In the case of the ferromagnetic materials used in monolithic magnetoresistances, the resistance varies with the angle formed by the direction of the current density passing through the component and the magnetization direction in said material. The resistance is weakest when said angle is equal to .pi./2. It is therefore not of interest in this case to apply the magnetic field in any random direction, but perpendicular to the length of the magnetoresistive component. However, parallel to the length of the bar, there is no magnetoresistive effect within monolithic ferromagnetic materials.
In new MMMS's, the resistivity and therefore resistance variation is independent of the angle formed between the direction of the field present within the magnetoresistive component and the direction of the current flowing through it.
However, said field is equal to the difference between the demagnetizing field H.sub.d and the external magnetic field H applied. Bearing in mind the fact that the shape and size of the MMMS's are such that from a magnetic standpoint, said structures are generally anisotropic, the demagnetizing field is not identical in all directions in space. Thus, the sensitivity of the components varies with the direction of the magnetic field to be measured as soon as they have an anisotropy of form.
The invention is directed at a magnetoresistive component and at a transducer using said component optionally applicable to monolithic ferromagnetic materials and more particularly to metallic multilayer magnetic structures, having compared with the prior art components and transducers a reduction of the effects of the demagnetizing field and therefore an increase in sensitivity.